Mode selection in a flash memory device

ABSTRACT

A single flash memory device has selectable read modes for either a segment mode or a page mode. The desired mode is selected by writing a control word to a mode control register. Selecting the segment mode causes the device to output selected memory segments. Selecting the page mode causes the device to output selected memory pages.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates generally to memory devices andparticularly to flash memory devices.

II. Description of the Related Art

Flash memory devices have developed into a popular source ofnon-volatile memory for a wide range of electronic applications. Flashmemory devices typically use a one-transistor memory cell that allowsfor high memory densities, high reliability, and low power consumption.Common uses for flash memory include portable computers, personaldigital assistants (PDAs), digital cameras, and cellular telephones.Program code, system data such as a basic input/output system (BIOS),and other firmware can typically be stored in flash memory devices.

A synchronous flash memory device is a type of flash memory device thathas a synchronous dynamic random access memory (SDRAM) interface forread operations. This enables the synchronous flash memory device tooperate at much higher speeds than a typical flash memory by opening apage (i.e., a row) of 8000 bits at one time. This increases thesynchronous flash device's performance at the expense of higher currentusage.

FIG. 1 illustrates a simplified block diagram of a typical prior artsynchronous flash memory array (100) architecture. The memory array(100) generally consists of a number of memory array banks (102, 104,106, 108) that have rows (110) and columns (112). A memory bank (102,104, 106, 108), upon being accessed with a bank, row, and columnaddress, loads an internal data latch circuit (114) with a definednumber of bits.

When a bank and row are selected by an input row address, the contentsof the row of memory cells are placed on column bit lines coupled tosense amplifiers that are located in the latch circuitry block (114).The values detected by the sense amplifiers are latched into the latchcircuitry (114). An input column address, that defines the column page,selects from the latches of the latch circuitry (114) and, thus, fromthe columns of the active row page.

To encompass a large variety of applications, chip designers make somerestrictive choices in the characteristics of memory designs. Thesecharacteristics include current consumption and performance. Thedesigners typically have to trade off one for the other since the fasterthe memory device, the higher the current consumption. For example, anelectronic circuit that requires the speed of the synchronous flash hasto live with relatively high current consumption as well. Whereas anelectronic circuit that has the low current consumption of the virtualsynchronous flash may sacrifice its performance somewhat.

Battery powered devices would benefit more from low power consumptionthan higher performance. Line powered devices would benefit more fromfaster performance than low power consumption. In order to satisfy bothmarkets, flash memory designers typically have to design multipleversions of a memory. This requires more time and money on the part ofthe manufacturer. There is a resulting need in the art for a way tochoose power consumption versus throughput in a single flash memorydevice.

SUMMARY

The present invention encompasses a flash memory device having a flashmemory array that comprises a plurality of flash memory cells. A modecontrol register accepts a control word that selects between a page readmode and a portion/segment of page read mode.

In one embodiment, the flash memory device is coupled to a processorthat generates the control word. The processor, in this embodiment, alsogenerates the extended address lines required to access the segmentedsections of the memory array when the flash memory device is in thesegment read mode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified block diagram of a typical prior artsynchronous flash memory array architecture

FIG. 2 shows a block diagram of one embodiment of a memory system of thepresent invention.

FIG. 3 shows an operation of memory segment association of oneembodiment of the present invention of a synchronous flash memory devicehaving a segment read mode.

FIG. 4 shows formatting of one embodiment of a mode register of thepresent invention.

DETAILED DESCRIPTION

The embodiments of the memory device of the present invention provide aflash memory device that is selectable to perform a read operation ineither a page mode or a page segment mode. This is accomplished bywriting a predetermined command word or bit to a mode register to enablethe desired type of flash memory device characteristics for a particularelectronic design.

FIG. 2 is a functional block diagram of a flash memory device (200) ofone embodiment of the present invention that is coupled to a processor(210). The flash memory device (200) and the processor (210) may formpart of an electronic system (220). The flash memory device (200) hasbeen simplified to focus on features of the memory that are helpful inunderstanding the present invention. The flash memory device isselectable, through the control registers, to have the attributes ofeither a synchronous flash memory device that is read in a page mode ora synchronous flash memory device that is read in a page segment/portionmode.

The flash memory device includes an array of memory cells (230). Thememory cells are non-volatile floating-gate memory cells. The memoryarray (230) is arranged in banks of rows and columns.

An address buffer circuit (240) is provided to latch address signalsprovided on address input connections A0-Ax (242). Address signals arereceived and decoded by a row decoder (244) and a column decoder (246)to access the memory array (230). It will be appreciated by thoseskilled in the art, with the benefit of the present description, thatthe number of address input connections depends on the density andarchitecture of the memory array (230). That is, the number of addressesincreases with both increased memory cell counts and increased bank andblock counts.

Additional bank address lines (282) are used to access the differentbanks of the memory array (230). In one embodiment of the presentinvention, there are four memory banks. In such an embodiment, two bankaddress lines, BA1 and BA0, are required to activate each memory bank.For example, if memory bank 3 is desired to be activated, from memorybanks 0, 1, 2, and 3, then BA1=1 and BA0=1. If a memory embodiment hasdifferent quantities of memory banks, different quantities of bankselect lines may be required.

The flash memory device (200) reads data in the memory array (230) bysensing voltage or current changes in the memory array columns usingsense/latch circuitry (250). The sense/latch circuitry, in oneembodiment, is coupled to latch a row of data from the memory array(230). Data input and output buffer circuitry (260) is included forbi-directional data communication over a plurality of data (DQ)connections (262) with the processor (210). Write circuitry (255) isprovided to write data to the memory array.

A mode control register (280) stores a mode control word of the presentinvention. The mode control register (280) may be comprised ofadditional control registers that are used for typical control functionsand others are reserved for expansion and/or future use.

In one embodiment, the mode control register (280) is comprised of flashmemory cells or fuses and can be programmed only once, such as at themanufacturer. In such an embodiment, a customer may not be able toreprogram the flash memory device but the manufacturer can still reducethe number of parts it has to design by having both functions in oneintegrated circuit. In alternate embodiments, the mode control registercan be programmed by the customer only once or reprogrammed multipletimes depending on the flash memory device characteristics desired.

A command control circuit (270) decodes signals provided on controlconnections (272) from the processor (210). In one embodiment, thecontrol circuit (270) is comprised of a state machine that executes thefunctions of the memory array (230), including data read, data write,and erase operations. The state machine may also be responsible forexecuting the read functions required for either the page mode or thepage segment mode, depending on the control word.

In the embodiment illustrated in FIG. 2, the processor (210) generatesthe address, data, and control lines to the memory device (200).Alternate embodiments may use other controllers to generate thesesignals in an electronic system (220). Additionally, the memory device(200) may be coupled to something other than a controller or processorthat generates the address, data, and control signals.

The flash memory device illustrated in FIG. 2 has been simplified tofacilitate a basic understanding of the features of the memory. A moredetailed understanding of internal circuitry and functions of flashmemories are known to those skilled in the art.

FIG. 3 illustrates one embodiment of segment addressing in a flashmemory device of the present invention. FIG. 3 shows a memory bank (300)of a synchronous flash memory with four segments (302, 304, 306, and308) that has been logically re-mapped to an SDRAM configuration. A rowof the segment zero (310) is shown being logically repositioned by anaccess that includes an extended synchronous flash segment address,discussed subsequently, to appear as if it was placed in one of theother three segments of the current memory bank (segment one 312,segment two 314, or segment three 316). The ability of full segmentassociation allows any row selected in a bank to be addressed to any ofthe four sense amplifier/latch segments (320-323) in that bank. This hasthe effect of logically shifting the address space by that number ofsegments for a subsequent memory access.

Using the above-described segment association, the memory device onlyactivates 2000 sense amplifiers versus 8000 sense amplifiers for anSDRAM or page mode synchronous flash memory device. In addition, becausesixteen segments (i.e., 4 banks with 4 segments each) are available forthe virtual synchronous flash memory device versus four segments for anSDRAM, when each of the four banks has their respective four segmentsactivated, there is a higher hit rate for read operations withoutactivating new rows. The increased granularity of a segment mode flashmemory device decreases power usage and, in addition, decreases memoryaccess latency. This effectively increases bandwidth from the segmentmode synchronous flash device over that of a conventional page modesynchronous flash memory device.

The various memory segments (302, 304, 306, and 308) of the memory bank(300) illustrated in FIG. 3 are selected by two of the address linesthat are coupled to the memory device. In one embodiment, address linesA14 and A15 select which of the four segments (302, 304, 306, and 308)of the currently active memory bank (300) are required for a particularoperation.

As an example of operation, assume an operation requires access tosegment 3 (308) of memory bank 2. Memory bank 2 is activated by BA1=1and BA0=0. The particular segment 3 is selected by A14=1 and A15=1.

Alternate embodiments may use different address lines to select aparticular segment. For example, larger memory devices may requireA0-A15 for memory addressing. In this case, address lines A16 and A17may be used for segment addressing.

For purposes of clarity, FIG. 3 illustrates only one bank of a virtualsynchronous flash memory device of the present invention. Alternateembodiments have multiple banks of memory cells that make up the memoryarray. Also, even though the embodiment of FIG. 3 shows four memorysegments, the present invention is not limited to any one quantity ofmemory segments.

As an example of operation with reference to FIGS. 2 and 3, addressvalues for the flash memory (200) are received from the processor (210)on the external address bus connections (242) and bank address lines(282). The received address values are stored internal to the flashmemory device (200) and used to select the memory cells in the internalmemory array (230). In a conventional SDRAM compatible operation, theprocessor (210) inputs a row and bank address on address lines A0-A11(242) and bank address lines BA0 and BA1 (282) thus activating aninternal virtual row page (not shown) in the flash memory device (200).

The row and bank address is followed by a “read” command with a columnaddress placed on address lines A0-A8 (a subset of address lines A0-Ax(242)) by the processor (210). Internal to the flash memory device(200), data values from the bank segments (not shown) are captured inthe latch circuits (250), logically re-mapped to form an SDRAMcompatible data format, and readied for transfer from the flash memorydevice (200).

Data transfer from the flash memory device (200) begins on the followingclock cycle and is transmitted on the bi-directional data word interface(262) to the processor (210). Control of the flash memory device (200)for operations is actuated by the internal control circuitry (270). Thecontrol circuitry (270) operates in response to external control signals(272) received from the processor (210) on control signal externalinterface connections (272) and to internal events.

Extended access to the flash memory device (200), when the device isoperating with segment mode characteristics, is accomplished utilizingthe extended synchronous Flash address lines, A12-A15 (a subset of A0-Ax(242)). The processor (210), with an “active” command, inputs a row onaddress lines A0-A11 (242), extended row and segment address linesA12-A15 (242), and bank address lines BA0 and BA1 (282). The “active”command directly activates a bank and internal row page and/or segmentof the internal memory array (230) in the flash memory device (200) forfurther operations.

The row, extended row, and bank address are followed by a “read” commandwith a column address placed on address lines A0-A8 (a subset of addresslines A0-Ax (242)) by the processor (210). If the memory device isoperating with segment mode characteristics, data values from the banksegments (not shown) are readied for transfer from the synchronous flashmemory device (200) by being copied into the internal latch circuits(250) as illustrated in FIG. 3.

Data transfers from or to the flash memory device (200) begin on thefollowing clock cycle received and are transmitted on the bi-directionaldata word interface (262) to the processor (210). Control of the flashmemory device (200) for operations is actuated by the internal controlcircuitry (270). The control circuitry (270) operates in responseexternal control signals received from the processor (210) on controlsignal external interface connections (272) and to internal events.

FIG. 4 illustrates one embodiment of a mode register of the presentinvention. Writing a command word to one of possibly multiple locationsof the mode control register (i.e., 280 in FIG. 2) sets the operatingmode of the memory device.

As one example of operation, the illustrated register format of the modecontrol register of FIG. 4 enables the burst length (401) to be set bybits M0-M2. As is well known in the art, the burst length determines themaximum number of column locations that can be accessed for a given reador write command.

Bit M3 (403) sets the burst type. In one embodiment, the burst typemight be sequential while another embodiment chooses the burst type tobe interleaved.

The CAS latency is chosen by bits M4-M6 (405). CAS latency is the delay,in clock cycles, between registration of a read command and theavailability of the first output of data. In this embodiment, thelatency can be set to one, two, or three clock cycles.

The operating mode of the memory device is set by bits M7 and M8 (400).These bits (400) determine the memory device's characteristics. In oneembodiment, M7=0 and M8=1 set the memory device's characteristics to bethat of a segment mode synchronous flash memory device. If M7=1 andM8=0, the memory device has the characteristics of a page modesynchronous flash memory device. An alternate embodiment reverses thevalues of these bits to select the desired device characteristics. Stillanother embodiment uses different values and/or different bit locationsin the mode control register to select the device characteristics.

Bits M9-M11 (410), in one embodiment, are reserved. These bits may bereassigned for other memory functions in other embodiments of the memorydevice.

The format of the mode register illustrated in FIG. 4 is for purposes ofillustration only. The device type bit can be located in any location ofthe mode register. In one embodiment, the device type bit is in adifferent location of the mode register than the other selectablefunctions that are initialized in this register. In such an embodiment,the mode is selected by writing a logical 1 or 0 in the appropriate bitlocation of the register.

In one embodiment, the page mode synchronous flash memory functions andthe segment mode flash memory functions share substantially the samefunctional circuitry. Alternate embodiments use separate circuitry on asingle integrated circuit for each function.

In summary, the embodiments of the present invention enable anintegrated circuit manufacturer to reduce the quantity of partsnecessary to support both high performance and low power flash memorydevices. This is accomplished by incorporating both page modesynchronous flash memory and segment mode synchronous flash memorycharacteristics into one integrated circuit. These characteristics arethen user selectable by initializing the mode control register with theproper control word.

Numerous modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

What is claimed is:
 1. A flash memory device comprising: a flash memoryarray comprising a plurality of flash memory cells; a mode controlregister capable of accepting a mode control word; and control circuitrythat selects either a page read mode or a segment read mode in responseto the mode control word.
 2. The flash memory device of claim 1 andfurther including a plurality of sense amplifiers coupled to the flashmemory array.
 3. The flash memory device of claim 2 wherein, in thesegment read mode, the flash memory array and the plurality of senseamplifiers are grouped into a plurality of segments such that any one ofthe flash memory segments can be associated with any one of theplurality of sense amplifier segments.
 4. The flash memory device ofclaim 1 wherein the flash memory array is comprised of a plurality ofbanks of memory.
 5. The flash memory device of claim 3 and furtherincluding a plurality of extended address inputs that select from amongthe plurality of flash memory segments in response to address signalinputs.
 6. The flash memory device of claim 1 and further including astate machine that controls memory functions of the flash memory device.7. The flash memory device of claim 1 and further comprising a pluralityof latch circuits grouped into a plurality of segments.
 8. A flashmemory device comprising: a flash memory array comprising a plurality offlash memory cells; page mode synchronous flash memory functionalcircuitry; segment mode synchronous flash memory functional circuitry;and a mode control register capable of accepting a control word thatselects between the page mode synchronous flash memory functionalcircuitry and the segment mode synchronous flash memory functionalcircuitry.
 9. The flash memory device of claim 8 and further including:a plurality of sense amplifiers coupled to the flash memory array, theplurality of sense amplifiers capable of being grouped into a pluralityof segments in response to the control word selecting the segment modesynchronous flash memory functional circuitry, each segmentcorresponding to a segment of the flash memory; and a plurality oflatches, each latch coupled to a different sense amplifier.
 10. Theflash memory device of claim 8 and further including a state machinecoupled to the mode control register, the state machine capable ofinterpreting the control word and selecting the synchronous flash memoryfunctional circuitry or the virtual synchronous flash memory functionalcircuitry in response to the control word.
 11. The flash memory deviceof claim 8 wherein the mode control register is reprogrammable.
 12. Theflash memory device of claim 8 wherein the mode control register iscomprised of non-volatile memory cells that are programmable only once.13. A flash memory device comprising: a flash memory array comprising aplurality of flash memory cells; a control circuit having a selectablesynchronous flash memory function and a virtual synchronous flash memoryfunction; and a mode control register that selects one of thesynchronous flash memory function or the virtual synchronous flashmemory function in response to a control word written to the modecontrol register.
 14. The flash memory device of claim 13 wherein thecontrol circuit comprises a state machine that executes the selectedsynchronous flash memory function or the virtual synchronous flashmemory function in response to the control word.
 15. A method forselecting one of a page read mode or a segment read mode in a flashmemory device, the method comprising: accepting a control word in a modecontrol register; and executing either the page read mode or the segmentread mode in response to the control word.
 16. The method of claim 15and further comprising reprogramming the control word.
 17. The method ofclaim 15 wherein the control word is received from a processor.
 18. Amethod for selecting between flash memory characteristics in a flashmemory device, the method comprising: receiving a control word thatindicates a page mode or a segment mode read characteristic; storing thecontrol word in a mode control register; determining the page mode orthe segment mode read characteristics selected by the control word; andexecuting flash memory functions consistent with the selected readcharacteristics.
 19. An electronic system comprising: a processor forgenerating a mode control word and address signals; and a flash memorydevice comprising: a flash memory array comprising a plurality of flashmemory cells; a mode control register capable of accepting the modecontrol word; and control circuitry that selects either a page read modeor a segment read mode in response to the mode control word.
 20. Theelectronic system of claim 19 wherein the address signals are used toselect segments of the flash memory array if the segment read mode isselected.